1. Field of the Invention
The present invention is directed to a method and apparatus for monitoring the operating condition of an electro-optical transmission system.
2. Description of the Related Art
Electro-optical systems for the transmission of PCM signals usually employ optical receivers which utilize a photodiode to convert the optical input signal to an electrical signal. The electrical signal is subsequently amplified and converted into a serial data signal in a time and amplitude decision unit. The output of the time and amplitude decision unit is monitored to detect the occurrence of a frame identifier word. If a frame identifier word is not detected after a specified period of time, a loss of synchronization can be reported.
FIG. 1 is a basic schematic representation of an electro-optical transmission and receiving system that is commonly employed in the art. The transmission terminal 1 uses an electro-optical converter 4 to transmit light pulses along transmission line 2. The receiving line terminal 3, utilizes an opto-electrical converter 5 to convert the optical signals received along line 2 into electrical signals. These electrical signals are amplified in an amplifier 30 and supplied to the rest of the receiving line for further processing.
The transmission characteristics of a digital data link in an optical telecommunications system can be monitored at the receiving line terminal. An optical and/or electrical alarm can be triggered when given threshold characteristics of the digital data link are exceeded. This manner of monitoring the digital data link characteristics is shown, for example, in the periodical Telcom Report Spezial, "Multiplex-und Leitungseinrichtungen", Siemens, March 1987, pages 97-8.
The optical input signal level to the receiving line terminal can be monitored by tapping electrical signals from various locations in the electrical signal path, including at the gain control circuits which regulate the amplification of the optical input signal after it has been converted to an electrical signal by the photodiode. Based on the monitored signal, a decision can be made as to whether or not an optical input signal is indeed present. This determination is made by comparing the observed signal to a threshold in a threshold comparator.
Most telecommunication systems employ extremely long transmission lines. Consequently, the input to the opto-electric transducer (i.e., photodiode) at the receiving line terminal is often substantially attenuated. As a result, the construction and tuning of the threshold comparators becomes quite complex because the difference between a "light" and "no light" condition at the input is extremely small. Additionally, the strength of the optical input to the opto-electric transducer is highly dependent on weather-related conditions that have an effect on the overall cable system. Changes in the transmission terminal (i.e., changes in the characteristic of the transmission laser) can also cause fluctuations in the optical input signal level to the receiving line terminal. Signal to noise ratios also play a role in the construction and tuning of the threshold comparators since the maximum gain of the amplifiers within the receiving line terminal is usually set without applying an input signal. As a result, any excessive gain might cause the noise present at the receiving line terminal input to falsely trigger data signals within the receiving line terminal.
As a result of these difficulties, either the presence of the digital signal or the presence of a frame identifier word is checked in the receiving line terminal. The lack of a frame identifier word indicates an "asynchronous" data condition. However, the mere presence of an asynchronous data condition does not assist in determining whether the transmission link is interrupted or whether the transmission or receiving terminal has experienced a malfunction.